For example, there is known a super-channel in which a plurality of sub-channels undergo high-density wavelength multiplexing and are thereby regarded as one signal, the wavelength multiplexing using an orthogonal frequency division multiplexing (OFDM) technique or a Nyquist-wavelength division multiplexing (WDM) technique.
Japanese Laid-Open Patent Publication No. 2013-106328 is an example of related art.
In a case where a free frequency band exists between a super-channel transmitted through an optical transmission line and another channel adjacent to the super-channel, a sub-channel (SC) on the free frequency band side of the super-channel is influenced by nonlinear noise, and thus there arise differences in transmission performance among SCs. As a result, the transmission characteristics of the entire super-channel are deteriorated. Moreover, the transmission performance differences among the SCs vary with the width of the free frequency band.